Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers.
One particular characteristic of a personal audio device, which may affect its marketability and desirability, is the dynamic range of its audio output signal. Stated simply, the dynamic range is the ratio between the largest and smallest values of the audio output signal. One way to increase dynamic range is to apply a high gain to the power amplifier. However, noise present in an audio output signal may be a generally monotonically increasing function of the gain of the power amplifier, such that any increased dynamic range as a result of a high-gain amplifier may be offset by signal noise which may effectively mask lower-intensity audio signals.
U.S. patent application Ser. No. 14/083,972, filed Nov. 19, 2013, entitled “Enhancement of Dynamic Range of Audio Signal Path,” and assigned to the applicant (Cirrus Logic, Inc.) of the present disclosure (the “'972 Application”) discloses methods and systems for enhancing the dynamic range of an audio signal path. In the '972 Application, an apparatus for providing an output signal to an audio transducer includes a digital signal path portion, an analog signal path portion, a digital-to-analog converter (DAC) interfaced between the digital signal path portion and the analog signal path portion, and a control circuit. The digital path portion may have a selectable digital gain and may be configured to generate a digital audio output signal in conformity with the selectable digital gain, and the DAC may be configured to generate an analog signal from the digital output signal. The analog signal path portion may have an audio input for receiving the analog signal, an audio output for providing the output signal, and a selectable analog gain, and may be configured to generate the output signal based on the analog signal and in conformity with the selectable analog gain. The control circuit may be configured to select the selectable analog gain and select the selectable digital gain based on a magnitude of a signal indicative of the output signal.
Effective operation of a dynamic range enhancement system often requires passband flatness of a signal path over a frequency range of interest, as deviations from a flat passband can lead to false triggering of the dynamic range enhancement system to alter the selectable analog gain and the selectable digital gain. However, each of an analog signal path portion and a digital path portion of a signal path may have transfer functions which are not flat over a frequency range of interest, a condition often referred to as “magnitude droop.”
The power amplifier may often be the primary consumer of power in a personal audio device, and thus, may have the greatest effect on the battery life of the personal audio device. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, because the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Therefore, amplifier topologies such as Class-G and Class-H are desirable for reducing the voltage drop across the output transistor(s) and thereby reducing the power wasted in dissipation by the output transistor(s).
In order to provide a variable power supply voltage to such a power amplifier, a charge pump power supply may be used, for example such as that disclosed in U.S. Pat. No. 8,311,243, in which an indication of the signal level at the output of the circuit is used to control the power supply voltage in a Class-G topology. The above-described topology may raise the efficiency of the audio amplifier, in general, as long as periods of low signal level are present in the audio source. Typically in such topologies, a plurality of thresholds define output signal level-dependent operating modes for the charge pump power supply, wherein a different supply voltage is generated by the charge pump power supply in each mode.